An example of a measuring instrument wherein the quantity to be measured exercises direct influence on the condition of the material of the sensor element is the resistance thermometer. In such instruments, the sensor element is traversed by a current. The amount of current flowing through the sensor element is directly dependent upon the ambient temperature of the sensor element, and can thus be used as an analogue for the temperature to be measured. Thus, in this measuring instrument, the measured quantity has a direct influence on the thermic condition of the material of the sensor element.
By contrast, the thermal conduction vacuum meter, commonly known as a Pirani gauge, is an example of a measuring instrument wherein the quantity to be measured exercises indirect influence on the condition of the material of the sensor element. In this instrument, the relationship between the pressure of a gas and its thermal conductivity is exploited for measuring pressure. A current traversed sensor element emits more heat when subject to higher pressures. The level of flowing current is dependent on the thermic condition of the sensor element, so that this condition can be employed as a measure for the pressure. In a measuring instrument of this type, the measured quantity (i.e., pressure or vacuum) has an indirect influence on the thermic condition of the sensor element.
The above described sensors are relatively technologically complex. In particular, thermal conduction vacuum meters have a limited sensitivity, and thus a limited range of measurement, at high and extremely low pressures.
It is also known to use sensor elements that are composed of a deformation heat-recoverable material for observing temperature. Materials of this type are known from "Legierungn mit Formgedaechtnis" Dieter Stoeckel, Expert Verlag, 1988. This text describes alloys whose atoms have different crystalline structures at different temperatures, more specifically an Austenite structure at higher temperatures (at a high-temperature phase) and a Martensite structure at lower temperatures (a low-temperature phase). Materials of this type "remember" a form impressed in the high-temperature phase. When these materials are deformed in a low-temperature phase, and then subsequently brought into a high-temperature phase by heating, they re-assume their original form that was impressed in the high-temperature phase. The change in shape or "recovery" begins at what is referred to as the A.sub.s -temperature, i.e., a temperature at which the structural transition from low-temperature phase into high-temperature phase begins. The overall recovery occurs within a relatively small temperature range, for example 10 to 20 K. Dependent upon the specific alloy, the A.sub.s -temperature can lie between -150.degree. and +150.degree. C.
Objects formed from deformation heat-recoverable materials thus have the property that a deformation (bending, torsion, dilatation, or compression) carried out at low-temperatures (below the A.sub.s -temperature) is "undone" when the object is heated to reach a high-temperature phase. For example, a wire composed of the material Nitinol stretched at low-temperatures shortens by about 3 to 5% upon transition from the low-temperature phase into the high-temperature phase. This change in length is not only directly opposite the "normal" thermic change in length, but is also greater by a multiple.
As a result of the relatively small temperature range in which recovery occurs, deformation heat-recoverable materials have heretofore been employed in temperature measuring devices only to an extremely limited degree. They are therefore used as temperature "detectors", and essentially serve a temperature monitoring function. Their job is to generate a signal when a defined temperature is upwardly or downwardly exceeded (see Patent Abstracts of Japan, Vol. 8, No. 259, JP-A-59-131 130, U.S. Pat. No. 3,483,748, and French Patent No. 22 85 601).